1. Field of the Invention
The invention relates to analytical methods for measuring low levels of peroxyacetic acid (also known as peracetic acid) in water.
2. Description of the Related Art
Peroxyacetic acid (PAA)-hydrogen peroxide (HP) equilibrium products are available from several manufacturers in the United States. The products are currently under consideration as chlorine alternatives in wastewater disinfection when applied in the range of about 1-10 ppm as PAA. In order for these products to be permitted in municipal wastewater applications, the results of aquatic toxicology tests, efficacy tests and field trials result must be presented to government regulatory agencies. Without a simple, accurate, reliable, and readily-available test procedure for PAA, the results of such tests would be meaningless. The various regulatory agencies require disinfectant-type products to be measured and monitored routinely during the treatment process so as to gauge the effectiveness of the treatment and estimate the impact of the chemical on the environment.
PAA-HP equilibrium products are also dosed to several other water systems in the 1-10 ppm range for control of bacteria, algae, fungi, and slime in recirculating cooling towers, reverse osmosis, and ultra filtration systems. Minimum levels of PAA must be maintained in order to ensure that the anticipated disinfection results are achieved. Here again, there is a need for a simple, accurate, and inexpensive test procedure. In addition, because the test results are used to determine the feed rates, an erroneous or unreliable test method may increase the cost to the end user by unnecessarily requiring the use of more PAA. Therefore, accurate and reliable testing methods are important to control costs, as well as to assure system integrity and to protect and monitor our environment.
There are several different types of reagent test kits available to determine PAA in water. Some are based on test strips consisting of a pad impregnated with an indicator reagent that changes color in response to the concentration of the PAA analyte. The user dips the test strip in the water for a few seconds and matches the color response to a comparator chart corresponding to the analyte concentration. Test strips suffer notorious deficiencies well known to those skilled in the art. The indicator reagent may leach from the pad when immersed in water to make color comparison impossible. Any color that does develop is strongly dependent on the length of time the strip is immersed, and the length of time after immersion until the color comparison is made. In addition, because the test strip method is a subjective test, its accuracy is limited. Consequently, although test strips are convenient to use, they are not accurate enough for reliable measurements.
Another test kit is based on the ceric IV sulfate-sodium thiosulfate titration reaction that employs a ferroin indicator. This technique can only be used when the concentration of the PAA is greater than 30 ppm, making it unsuitable for the low levels necessary in water, wastewater, and recirculating cooling water systems.
A commercially available test kit from CheMetrics based on dimethyl-substituted N,N-diethyl-p-phenylenediamine (DDPD) boasts a much lower detection limit of 0.5 ppm PAA. In this method, the user treats the sample with an excess of potassium iodide. A one-minute reaction time permits the PAA to oxidize the iodide to iodine. The sample is then introduced to a solution of DDPD indicator, which forms a purple color in direct proportion to the PAA concentration. Color comparator tubes or a pre-calibrated spectrophotometer set to 565 nm are used to quantitate the amount of PAA present. This method has a number of limitations. For example, although it provides reliable and reproducible results in deionized water, the opposite is true in natural waters, even clean drinking water. It is uncertain why this method exhibits these irregularities. It may be related to underbuffering of the DDPD solution which causes the pH to swing outside the 6.2-6.5 range for optimum development of the indicator. The results are therefore unreliable for any diluted solution of peroxyacetic acid, with the exception of deionized or distilled water.
Another method, developed by Wagner et al., Water Environment Research, vol. 74, p. 33 (2002), uses the 2,2′-azino-bis(3-ethylbenz-thiazoline-6-sulfuric acid) diammonium salt (ABTS)-horseradish peroxidase (HRP) assay with a spectrophotometer set to 405 nm. This method is accurate and reliable, but is quite cumbersome and difficult to perform, and requires reagents that are expensive and difficult to obtain. This procedure is not amenable for field use or manipulation due to its complexity, costs, time requirement to perform the analysis, and availability of reagents. In addition, any reading over 1 mg/l must be diluted and analyzed again to obtain solutions <1 mg/l.
A common method for analyzing low levels of chlorine in treated waters is the N,N-diethyl-p-phenylenediamine (DPD) indicator-colorimetric method approved by the United States EPA. To measure free chlorine, the DPD indicator is introduced to the water along with a buffer system to adjust the pH to 6.2-6.5. Free chlorine instantly oxidizes the DPD indicator to give a pink coloration, the intensity of which is proportional to the amount of free chlorine in the sample. To measure total chlorine, an iodide ion catalyst and a two-minute reaction time are required to quantitatively liberate iodine, the species that oxidizes the DPD indicator to the pink coloration. A spectrophotometer is then used that is programmed to measure the intensity of the pink coloration and display the results in terms of ppm free or total chlorine. Alternatively, the intensity of the coloration may be measured using a color comparator method that yields a result to corresponding to the chlorine concentration.
Despite the similar chemical properties of PAA and chlorine, however, the standard DPD indicator-colorimetric method has been found unsuitable for determining low levels of PAA. Unlike free chlorine, PAA does not instantly oxidize the DPD indicator when introduced to the water sample with a pH buffer. Moreover, when the PAA solution is exposed to the DPD indicator, pH buffer, and iodide ion combination, the hydrogen peroxide that always accompanies PAA oxidizes the DPD indicator to a significant extent during the two-minute reaction time. The result is a large false positive interference to the PAA response. Certainly, the hydrogen peroxide interference was recognized by Bolognesi et al., Science of the Total Environment, vol. 333, p. 127 (2004), who removed the hydrogen peroxide interference by pretreating the sample with a catalase enzyme and potassium iodide followed by use of a “total DPD reaction” for determining the PAA concentration.
Thus, there is a need for a method of analyzing a low level of PAA that is simple to use, accurate, and reliable. The method should use readily-available inexpensive reagents, and be amenable to packaging in the form of a portable kit for real-time measurements in the field. This invention addresses those needs.